Auto merge of #8037 - djc:rfc-2495, r=ehuss

Auto merge of #8037 - djc:rfc-2495, r=ehuss

Implement support for rust-version field in project metadata

Needs a bunch more work, but I'd like some early feedback! Remaining work:

- [x] Bikeshedding name (picked `rust-version` here over `msrv` or `min-rust-version`)
- [x] Failing test for local dependency with unsatisfied MSRV
- [x] Requirement cannot be smaller than 1.27
- [x] Requirement cannot be smaller than initial release of edition used
- [x] Check for `run`, `verify` and `publish` subcommands
- [x] More tests (would love feedback on this)
- [x] Handle pre-release identifiers
- [x] Disallow semver range operators
- [x] Feature gate it
- [x] Add documentation for unstable feature

Minimal version of `@moxian's` earlier take in #7801.

cc rust-lang/rust#65262